1. Field of the Invention
The present invention relates to a unique class of chiral compounds for liquid crystals, and to liquid-crystalline mixtures containing such chiral compounds, and to their use for cholesteric displays and the resulting devices.
2. Description of the Related Art
Cholesteric flat-panel displays are currently under development because of their low power consumption, bright viewing characteristics at wide angles, and high-resolution capability at low cost. Their low-power consumption is a result of the bistable memory characteristic inherent in the technology. As described in the first U.S. patents on this technology (See U.S. Pat. Nos. 5,251,048, 5,384,067, 5,437,811, and 5,453,863), each pixel of the display can exist in a stable color reflective state with any desired reflective intensity or brightness (gray-scale) without any applied power. The degree of brightness is electronically selected by a pulse. A unique feature described in those inventions, is the existence of a threshold in the electronic response to a pulse such that a matrix of pixels can be multiplexed to achieve a high resolution display at low cost without the need of transistor elements (active matrix) at each pixel. Because of the low power and reflective brightness characteristics, cholesteric displays are used in portable or handheld applications where long battery life and versatile viewing capabilities are important.
The reflective properties of cholesteric liquid crystals have been generally known for many years. Sometimes called a chiral nematic, a cholesteric liquid crystal achieves its color reflective property because the molecules are arranged in a helical twist pattern with a periodicity (pitch length) equal to the wavelength of light in the material. The first materials explored with this property were the cholesterol esters. These materials are not only chiral but also liquid crystalline and reflect iridescent colors when the periodicity of the twist corresponds to a reflective wavelength from 400 nm to about 700 nm [xe2x80x9cCholesteric Structurexe2x80x94II: Chemical Significancexe2x80x9d, p105-119, J. L. Fergason, N. N. Goldberg, R. J. Nadalin, Liquid Crystals, Ed. G. H. Brown, G. J. Dienes, M. M. Labes, Gordon and Breach Science Publishers, New York (1966)]. The materials were therefore called cholesteric liquid crystals, a name still used today even though cholesterol materials are seldom used today. Instead, a mixture of chiral and achiral compounds is used as discussed by Gottarelli et al. [xe2x80x9cInduced Cholesteric Mesophases: Origin and Application, G. Gottarelli, G. P. Spada, Mol. Cryst. Liq. Cryst., 123, 377-388 (1985)]. Achiral liquid crystalline compounds make up a nematic liquid crystalline host mixture, which has no helical twist by itself. To this host nematic is added a chiral compound to twist up the nematic material into one of a cholesteric structure, hence the name chiral nematic.
The helical arrangement of the molecules provides a self-assembled stack of dielectric layers because of the anisotropy of the refractive index of the molecules. The index of refraction continuously varies along the stack by as much as 0.25 depending upon the nematic material. Because of the helical nature of the refractive index in the layer, the stack will reflect one circular component of a selected bandwidth of colored light. A right handed twisted planar texture will therefore decompose incident unpolarized white light into its right and left components by reflecting the right hand component and transmitting the left. A left-handed twisted material will do the opposite. A left-hand display cell stacked on top of a right-hand cell, both with the same pitch length, will reflect all of the incident light.
According to Bragg""s law, the wavelength xcex, of the selective reflection is given by the equation: xcex=np where p is the pitch of the helical structure and n is the average refractive index of the liquid crystal mixture. In mixtures of a nematic liquid crystal with the chiral additive, the reciprocal of the pitch length is approximately proportional to the concentration X, of the chiral compound, pxe2x88x921=xcex2X with xcex2 being the helical twisting power (HTP). Conventional chiral additives available today have twisting powers typically of xcex2 less than 15 xcexcmxe2x88x921 when X is measured in weight percent.
Certain dimethanoldioxolane derivatives have been described in the literature as possessing large HTP values. E.g., xe2x80x9cTADDOLs with Unprecedented Helical Twisting Power in Liquid Crystalsxe2x80x9d, H. G. Kuball, B. Weiss, A. K. Beck, D. Seebach, Helvetica Chimica Acta, 80, 2507-2514 (1997); xe2x80x9cTADDOLs Under Closer Scrutinyxe2x80x94Why Bulky Substituents Make it All Differentxe2x80x9d, A. K. Beck, M. Dobbler, D. A. Plattner, Helvetica Chimica Acta, 80, 2073-2083 (1997); and in U.S. Pat. No. 5,637,255. Like other chiral additives, they are also known to generally possess large temperature dependent values of dp/dT and, hence, dxcex/dT. Their temperature dependencies tend to be positive in that the pitch length increases with w increasing temperature, causing a cholesteric material to change from blue to red reflecting. Furthermore, the temperature dependence dp/dT has been shown to depend on the material of the host nematic. See xe2x80x9cTADDOLs with Unprecedented Helical Twisting Power in Liquid Crystalsxe2x80x9d, H. G. Kuball, B. Weiss, A. K. Beck, D. Seebach, Helvetica Chimica Acta, 80, 2507-2514 (1997).
In order for flat-panel displays to be useful for portable applications, it is necessary for the display be operable over a wide range of temperatures. Outdoor temperatures can range from xe2x88x9220xc2x0 C. to +50xc2x0 C. depending on the environment. It would be advantageous if the reflected colors did not change over this temperature range. Implementing this desirable characteristic is not a straightforward task, since nearly all cholesteric liquid crystalline materials are well known to exhibit reflective colors that vary strongly with changes in temperature. Depending on the shape or chemical structure of the chiral molecule, the pitch length p and, hence, the peak reflective wavelength xcex, can increase with temperature (+dxcex)/dT) or decrease with temperature (xe2x88x92dxcex/dT). Also, in many cases, dxcex/dT is not linear over the temperature range of the cholesteric phase.
It should be mentioned at this point that the measurement for the temperature dependence of the pitch is performed using test cells, each of which is 5 xcexcm thick, and has a hard coat layer on both substrates. There is also an unrubbed polyimide surface on top of the hard coat layers. The measurement for the temperature dependence of the pitch is performed in the following manner. A collimated light is incident on the display at surface normal and the reflected light is detected at 45xc2x0. The display is scanned in the wavelength band that includes the peak-reflected wavelength, e.g., 400 nm to 700 nm, or 700 nm to 1500 nm. The cell is switched to the planar texture at each test temperature. The measurement is performed from xe2x88x9220xc2x0 C. to +70xc2x0 C. in 10xc2x0 C. intervals. It should be noted that, for the purposes of a flat temperature dependence, only a portion of the test temperature range, i.e., temperatures between +10xc2x0 C. to +50xc2x0 C., are considered. In any event, the maximum change in peak reflected wavelength is then recorded. A mixture is considered to exhibit temperature independent color behavior if the maximum change in the peak reflection wavelength is 30 nm or less across the temperature range of +10xc2x0 C. to +50xc2x0 C.
It has been shown that cholesteric displays fabricated using a mixture of two or more chiral compounds as additives can be made to produce a helical twist power and, hence, reflective wavelength that is independent of temperature by combining a chiral compound that has a +dxcex/dT with one with a xe2x88x92dxcex/dT. U.S. Pat. No. 5,309,265 describes a means of achieving a temperature independent xcex by combining a plurality of chiral compounds, where both compounds exhibit the same twist sense.
It will be noted that there are a number of factors that preclude employing many previously known chiral compounds in liquid crystals. First of all, a chiral compound must be soluble in the nematic liquid crystal host material; many compounds are simply not soluble or only weakly soluble and, thus, cannot be used. In compounds that are soluble, they may adversely affect the nematic material by substantially reducing the temperature range of the liquid crystalline phase. If the chiral compound has an HTP that is too low, it may be necessary to add a large quantity of the additive which can dilute some of the desirable physical properties of the host nematic needed for the cholesteric display.
Consequently, a need still exists for a chiral additive, which is readily soluble in a nematic host mixture and which can be used individually (i.e., without needing to be combined with other chiral materials) for cholesteric displays that provides a high helical twisting power, substantially independent of temperature in operating ranges suitable for the cholesteric displays. What is also needed is a cholesteric display exhibiting a high helical twisting power, which is substantially independent of temperature in operating ranges suitable for the cholesteric displays, which display includes at least one chiral additive, which is readily soluble in a nematic host mixture. In some cases it would be desirable if the temperature dependence of the cholesteric display could be tailored by the addition of a second additive, e.g., either an achiral compound or a second chiral additive different from the first chiral additive, where the twist senses of the first and second chiral additives are opposite to one another.
The above and other objectives are fulfilled by the present invention which concerns a unique class of chiral compounds that can be used alone as the optically-active additive or dopant in nematic liquid crystals to achieve a reflective wavelength that is temperature independent or essentially temperature independent, i.e., a dxcex/dT approaching zero in value, yet without significantly reducing the temperature range of the liquid crystalline phase, or diluting or otherwise adversely affecting its needed physical properties for liquid crystal implementations.
The inventive compounds are derivatives of dioxolanes and have the molecular structure generally indicated in FIG. 1 and reproduced below as general formula I. 
The R, R1, R2 and R3 substitutions on the molecule of general formula I control the temperature dependence of the twisting power in the nematic host mixture in a surprisingly superior fashion. Generally, the R2 and R3 groups at the number 2 position of the dioxolane ring independently are hydrogen, methyl or another lower alkyl group, or a substituted aryl or biaryl unit, while the R1 groups independently each are a hydroxyl, alkoxy, aryloxy, or arylalkoxy group. The R groups in general formula I indicate a group of general formula II, which is as follows:
A1xe2x80x94[xe2x80x94Zxe2x80x94]qxe2x80x94A2xe2x80x94xe2x80x83xe2x80x83II 
where A1 is an aromatic group, an acyclic aliphatic group, or an alicyclic group (e.g., a mono- or polycyclic aryl group, a straight chain or branched chain alkyl group, an arylalkyl group, an arylalkenyl group, a cycloalkyl group, a cycloalkenyl group), and A1 can be a substituted or unsubstituted group. For example, A1 can be benzyl, cinnamyl, phenethyl, cyclohexyl, and the like. Z is a group selected from xe2x80x94Oxe2x80x94, xe2x80x94OCOxe2x80x94, or xe2x80x94Sxe2x80x94, and the coefficient q is 0 or 1. Z can also be (CH2)nO with the coefficient n being 0 to 5 and the coefficient q being 1. A2 is a bivalent radical of naphthalene that may additionally be substituted. Preferably, A2 is a bivalent radical (2,6- or 1,5-disubstituted) of naphthalene, which may be unsubstituted or substituted (e.g., methyl, cyano, halogeno, amino, nitro, or hydroxyl substituents). The ring structure of A2, or A1 if it is cyclic, optionally can be heterocyclic, such as by replacement of one or more CH member(s) of the ring structure with N, O and/or S (e.g., a bivalent radical of quinoline, xanthene, carbazole, or acridine).
In one preferred embodiment, each R substituent of general formula I is independently selected as: 
where R4 can be represented by general formula III as follows:
Yxe2x80x94[xe2x80x94Xxe2x80x94]nxe2x80x94[xe2x80x94Zxe2x80x94]qxe2x80x94xe2x80x83xe2x80x83III 
where n is an integer value of 0 or 1 or more, X is xe2x80x94CHxe2x95x90CHxe2x80x94CH2xe2x80x94, or xe2x80x94(CH2)mxe2x80x94 where m is an integer value of 1, 2, 3, or more. Y is a radical of an aromatic hydrocarbon, an acyclic aliphatic hydrocarbon, or an alicyclic hydrocarbon group (e.g., a mono- or polycyclic aryl group, a straight chain or branched chain alkyl group, a cycloalkyl group, a cycloalkenyl group, and the like), and Y can be a substituted or unsubstituted group. Z and q have the same respective meanings as defined above relative to general formula II.
In one preferred embodiment, R4 is an aryloxy, an arylalkoxy, an arylalkyleneoxy, or an arylalkenyleneoxy group. Examples of R4 advantageously may include, for instance, benzyloxy (C6H5xe2x80x94CH2xe2x80x94Oxe2x80x94), cinnamyloxy (C6H5xe2x80x94CHxe2x95x90CHxe2x80x94CH2xe2x80x94Oxe2x80x94), phenethyloxy (C6H5xe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94), and the like, where R1 is a hydroxy group and R2 and R3 are methyl groups.
Additionally, the specific structures of the four R groups present in formula I or FIG. 1 can be identical to each other or they can independently vary from each other within the guidelines indicated above and herein.
The inventive optically-active, chiral additives according to the formula of FIG. 1 contain a sufficiently high helical twisting power (HTP) so that only a relatively small amount of the chiral additive is required to twist the nematic phase to a pitch length where it reflects wavelengths in the visible or infrared portion of the electromagnetic spectrum. On the other hand, the HTP of the additive is not inordinately high either, so that the invention can avoid undesired gradients in concentration, which could cause undesired inhomogeneities in the display. The present invention thereby avoids the need to combine separate types of chiral compounds in order to achieve a very high degree of temperature independence. While the present invention does not necessarily exclude the possibility, and in fact does contemplate, use of mixtures of different inventive chiral compounds within the scope of the formula of FIG. 1, the important point is that the inventive chiral compounds can be effectively used singly in a liquid-crystalline nematic mixture deployed in a light modulating apparatus to avoid the need for chiral additive combinations.